1. Field of the Invention
The present invention relates generally to the field of expression of desired, protein-encoding genes in a host cell. More particularly, it concerns expression of desired heterologous, e.g., mammalian, protein-encoding genes in a host cell wherein the heterologous gene is operably linked to a heterologous promoter recognized by a heterologous RNA polymerase, wherein the gene encoding the heterologous RNA polymerase is expressed in the host cell under the control of a host cell promoter. It also concerns ectopic expression in a host cell of desired native genes operably linked to a heterologous promoter recognized by a heterologous RNA polymerase, wherein the gene encoding the heterologous RNA polymerase is expressed in the host cell under the control of a host cell promoter.
2. Description of Related Art
Recombinant DNA technology allows insertion of a desired heterologous protein-encoding gene, which may also be referred to herein as a "target gene," into a host cell and subsequent expression of the gene product of the protein-encoding gene.
U.S. Pat. Nos. 4,704,362; 5,221,619; and 5,583,013 (Itakura et al.) disclose a method for expressing, in a bacterial host, proteins that are "heterologous" with respect to the host, i.e., not ordinarily produced by the host, using expression control systems that are "homologous" to the bacterial host, i.e., systems that are ordinarily produced by the bacterial host in its untransformed state. There are various reasons to provide new methods for expressing heterologous or, less commonly, homologous proteins in microbial hosts using a promoter system that is heterologous with respect to the host. However, some heterologous promoter systems that are advantageous for this purpose may not be recognized by the host's RNA polymerases and hence, without more, are not able to drive such expression. This invention provides a method for making such heterologous promoter systems useful to drive expression of such proteins in microbial and other hosts, by providing an RNA polymerase capable of recognizing the heterologous promoter.
A disadvantage of the homologous regulon system is the promoter recognized by the host RNA polymerase would be expected to drive expression of genes native to the host cell in addition to the protein-encoding gene. The presence of host gene products may decrease the purity of the target gene product and may require time-consuming and expensive steps for the removal of the host gene products. Therefore, it is desirable to have a system for expressing a protein-encoding gene in a host cell wherein the protein-encoding gene is under control of a promoter heterologous to the host cell, and as a result the protein-encoding gene can be expressed to yield greater purity of gene product. Such a system may be termed a "heterologous regulon." In a heterologous regulon system, it is necessary to provide a heterologous RNA polymerase that recognizes the promoter heterologous to the host cell and drives expression of the protein-encoding gene.